1. Field of the Invention
The invention relates to directly heatable hollow semiconductor bodies useful in diffusion doping processes and somewhat more particularly to directly heatable hollow semiconductor bodies and methods of producing such bodies by thermal decomposition of gaseous semiconductor compounds on heated surfaces of a carrier member, which, after semiconductor body formation, is removed.
2. Prior Art
Quartz tubes or ampules are used as doping containers during diffusion of dopants into semiconductor elements and are heated in tube ovens to diffusion temperatures. The use of such quartz or even graphite tubes or ampules in diffusion doping processes is difficult since the semiconductor element being doped must be so disposed within the ampule as not to contact the ampule material, since otherwise contamination of the semiconductor element results. Further, quartz ampules are limited to diffusion temperatures below about 1200.degree. C., the softening temperature for quartz. Additionally, the use of quartz ampules or tubes for diffusion doping processes requires special diffusion ovens since it is impossible to heat quartz by either direct or induction heat.
German Pat. No. 1,809,970 (which corresponds to U.S. Pat. No. 3,781,152 and British patent specification No. 1,263,580) describes a hollow semiconductor tube useful in diffusion doping processes in place of quartz or graphite tubes, along with a method of producing such semiconductor tubes. Generally, the method comprises feeding a thermally decomposable gaseous semiconductor compound onto a heated outer surface of a carrier member, for example, composed of graphite, so that a layer of semiconductor material forms on such outer surface. After the desired layer of thickness is achieved, the carrier member is removed without destroying the formed semiconductor body. Such a hollow semiconductor body or tube can be exposed to higher temperatures than a quartz or graphite tube so that diffusion doping processes using semiconductor tubes may be greatly accelerated. Further, semiconductor elements being doped within such tubes can contact the tube walls without adverse effects. These hollow semiconductor tubes are utilized as diffusion furnaces by closing the ends thereof with plugs formed of semiconductor material and which have gas lines therethrough for passage of a gaseous doping material and carrier gas to the interior of the tube and in contact with a semiconductor element or wafer located therein. The semiconductor tubes are provided with a heating coil which heats the tube by radiant heat to a select diffusion temperature. The coil may also be supplied with HF-energy.
German Offenlegungsscrift 1,933,128 (which corresponds to British patent specification No. 1,282,363) describes a diffusion doping system wherein a gas impermeable crystalline semiconductor tube functions as a diffusion container. The container is heated by either applying a voltage directly thereto or by high-frequency energy. This tube is either provided with electrodes at its ends or is encompassed by an induction heating coil. In the embodiment where induction heating of the tube is utilized, the tube is provided with a ring of a good conductive material, such as graphite. In the embodiment where direct heating via a voltage is used, the voltage required to attain diffusion temperature is dependent on the conductivity of the semiconductor material and on the tube dimensions. The aforesaid Offenlegungsschrift suggests that the diffusion container be formed of a highly doped semiconductor material, which can be economically produced so that the voltage required during the start of the heating process can be fairly low. Once a certain temperature is attained, the conductivity of the tube becomes independent of the amount of dopant in the semiconductor material and primarily depends only on the dimensions of the tube.
The gas phase deposition process of producing diffusion tubes as described in the above prior art yields very pure gas impermeable semiconductor tubes, in particular silicon or silicon carbide tubes. Extremely pure tubes can only be heated with a direct current after a pre-heating process. Doped tubes do not require pre-heating and, as set forth in the heretofore mentioned German Offenlegungsschrift 1,933,128, may be heated directly. However, such doped tubes cause undesirable reaction between the dopant within the tube material and the semiconductor element being doped.